Echo is a problem related to the perceived speech quality in telephony systems with long delays, e.g. telephony over long distances or telephony systems using long processing delays, like digital cellular systems. The echo arises in the four-to-two wire conversion in the PSTN/subscriber interface. To remove this echo, echo cancellers are usually provided in transit exchanges for long distance traffic, and in-mobile services switching centers for cellular applications.
Due to the location of the echo canceller it is made adaptive; the same echo canceller is used for many different subscribers in the PSTN. This adaption is necessary not only between different calls, but also during each call, due to the non-fixed nature of the transmission network, e.g. phase slips, three-party calls, etc.
The adaption of the echo canceller needs to be controlled, since it must be inhibited during presence of near end side speech, otherwise the echo path estimate will be degraded. This leads to a conservative strategy with a well protected estimate. However, the adaption strategy cannot be too conservative, since this will degrade the performance of the echo canceller when a fast re-adaption is necessary due to a change in the echo path loop. To overcome the optimization problem, namely fast re-adaption when the echo path changes and stable echo estimate during double-talk, a configuration with two echo path estimates may be used. Echo cancellers using two filters for echo estimation have been described in K. Ochiai et al, "Echo Canceller with Two Echo Path Models", IEEE Transactions on Communications, 25 (6): 589-594, June 1977 and U.S. Pat. No. 3,787,645. One filter, commonly known as the foreground filter, is non-adaptive and used for obtaining the actual echo canceller output. The other filter, commonly known as the background filter, is continuously updated with some adaptive algorithm, typically a normalized least mean square (NLMS) algorithm. The coefficients from the adaptive background filter are then transferred to the foreground filter whenever the background filter is considered better in some sense.
Since the configuration described above only uses the non-adaptive foreground filter for echo canceller output, it is very important that the adaptive background filter is transferred when it performs better. However, due to problems, partly caused by the conservative algorithm that is used, this may not occur and echo cancellation may be inhibited.